With the advance of an optical transmission technology, a superhigh speed optical transmission system using high-speed optical devices and single mode optical fibers has been proposed to realize a large capacity transmission and a long distance transmission. In this situation, an optical transmission system is required to operate with high speed and to be stable in order to realize wide-band communication in which various information of image, data and voice are transferred. That is, in such a wide-band communication network, a trank transmission system of time-division multiplexing-transmission type has a capacity of multigigabit-per-second, so that an optical transmitter and an optical receiver used for the system is required to operate with high speed and to operate in wide-band.
Recently, an optical regenerative-repeater communication system in which a signal is entirely processed optically without conversion between optical and electrical signals has been studied in order to enhance efficicincy thereof.
A conventional optical regenerative-repeater includes an optical amplifier for reshaping an optical input signal by amplification, an optical timing circuit connected to the optical amplifier for extracting an optical timing signal, and an optical decision circuit connected to the optical amplifier and the optical timing circuit, respectively. The optical amplifier, the optical timing circuit and the optical decision circuit are connected by optical fibers each other, so that the optical regenerative-repeater system fully operates only by processing optical signals.
In the conventional optical regenerative-repeater system, when an input optical signal is supplied to the optical amplifier, the optical signal is amplified and supplied to the optical timing circuit and to the optical decision circuit, respectively. The optical signal is regenerated by the optical decision circuit in accordance with an optical timing signal supplied from the timing circuit.
According to the conventional optical regenerative-repeater chain system, however, there are disadvantages that in characteristics thereof are lowered as compared with a system which is partly controlled electrically, because the optical timing circuit which largely affects the characteristics of the system operates with optical signals. That is, in the conventional system, an optical timing signal supplied from the optical timing circuit is unstable due to practical properties of optical devices and stability thereof, so that timing jitter occurs in the optical timing signal and the jitter is accumulated in an output optical signal from optical repeater.
In the conventional optical regenerative-repeater system, a fixed gain is obtained. As a result, a gain is not varied dependent on the level fluctuation of optical input signals. Further, when no input signal is supplied from a fiber transmission line, noise is dominant in optical signals supplied from the optical timing circuit. As a result, noise is generated in the optical regenerative-repeater system resulting in an extraordinary state of an optical communication system.